1. Field of the Invention
The present invention pertains generally to the field of radiation detecting devices and, more particularly, to the field of radiation dosimetry devices, such as disclosed in U.S. Pat. Nos. 5,079,426 and 5,262,649, the disclosures of which are incorporated by reference herein.
2. Discussion of the Background
There are several instances in which dosimetry of ionizing radiation, such as diagnostic x-rays, megavoltage x-rays, electrons, protons, neutrons, heavy ions, pions, etc., is needed. Dosimetry includes the measurement and/or calculation of absorbed dose, also known as dose, where dose is defined as the energy absorbed per unit mass of the irradiated target material (for example a patient). In one common instance in modem radiotherapy, x-rays or some other type of radiation are generated and are directed onto a patient.
In external beam megavoltage radiation therapy, it is highly desirable that a maximum dose be delivered to a target volume containing a tumor and a minimum dose be delivered to the normal tissue surrounding the target volume. In this application, it is important to fully characterize the source of the radiation beam so as to be able to predict the amount of dose absorbed in the patient including both normal and cancerous tissue. For example, it is highly useful to be able to determine the dose as a function of depth into the patient. This depends upon, among other things, the energy of the radiation beam, the size of the radiation field, the distance to the surface of the patient from the radiation source, and the density of various anatomical structures. Similar issues of characterization and prediction apply in the case of non-medical applications where a knowledge of dose in an object made of a material other than human tissue is desired.
Characterization of the source of the radiation beam in medical applications is typically accomplished by performing measurements of dose in a material, such as water, whose radiological properties are equivalent to human tissue. In the case of non-medical applications a number of options are available for characterization of the source of the radiation beam. For example, measurements in water (or some water equivalent material) can also be performed which, when combined with calculational correction, give predictions for the value of dose in the object of interest. Another method involves performing measurements of dose in a material whose radiological properties are equivalent to those of the object in which the dose is to be determined.
Two commonly utilized systems for performing dosimetric measurements are described below.
A first system utilizes a water tank, for example of 40 cm.times.40 cm.times.40 cm, and an x-ray sensor within the water tank. In such a system utilizing a water tank, x-rays are emitted from a radiation source at a predetermined energy. The x-ray sensor within the water tank is then moved to various positions to take measurements of the received x-ray radiation. This information is then stored. Then, an x-ray radiation of different parameters is output from the imaging device, and the sensor is again moved to various positions within the water tank to detect the incident x-ray radiation. The parameters which are varied from the radiation source may include energy, size of field, distance from radiation source to sensor, etc. This operation is then continued until the desired characteristics of the x-ray radiation output from the radiation source are determined.
A second known dosimetry system involves positioning x-ray film between layers of some solid material whose radiological properties closely approximate those of human tissue. We define the term human tissue substitute to describe such solid material. Examples of human tissue substitute include acrylic, plexiglass, or solid water (the latter made by Gammex, RMI) which are used as general substitutes for liquid water of which human tissue largely consists. Similarly, other materials are used as human tissue substitutes for lung, bone, etc. With this system, x-ray radiation is output from a radiation source while the x-ray film is positioned at a particular distance away from the radiation source, i.e., in a particular plane away from the x-ray source. Then, for a given set of parameters, e.g., energy, field size, distance from radiation source, etc., the x-ray film is exposed to the x-ray radiation and the x-ray film is then developed. This operation is then repeated for x-ray radiation of different parameters based on varying the parameters noted above, e.g., energy, field size, distance from radiation source to x-ray film, etc. All of the information from each of the x-ray films are then scanned and stored in a memory, and are then processed appropriately to determine the characteristics of the x-ray source.
This dosimetry process of determining the characteristics of the x-ray radiation on x-ray exposure film is very slow since the x-ray film must be variously exposed, and then information from the x-ray films must be scanned, digitized and processed.
U.S. Pat. Nos. 5,079,426 and 5,262,649 disclose radiation imaging systems which can be utilized for dosimetry. The systems disclosed in these patents require that an input x-ray radiation is converted into light energy by a phosphor scintillating layer 44 and a phosphor photo-to-electron conversion layer 46. The drawbacks with the dosimetry sensors disclosed in these devices is that the dosimetry sensors themselves which include the phosphor scintillating layer 44 and the photon-to-electron conversion layer 46 utilize high atomic number elements.